The providers of current day multiple channel wireless communication services such as cellular mobile telephone (CMT) and personal communication systems (PCS) typically allocate receiver signal processing equipment for each single receiver channel. For example, each basestation is configured to provide communication capability for only a limited number of the channels in the overall frequency spectrum that is available to the service provider.
A typical basestation may thus contain several racks of equipment which house multiple sets of receiver and transmitter signal processing components that service a prescribed subset of the available channels. For example, in an Advanced Mobile Phone Service (AMPS) cellular system, a typical basestation may service only selected number of channels, such as 48, of the total number, such as 416, of the channels available to the service provider.
There recently has been a suggestion that service providers would prefer to employ equipment that would be more flexible, both in terms of where it can be located, as well as in the extent of the available bandwidth coverage provided by a particular transceiver site. This is particularly true in rural areas where cellular coverage may be concentrated along a highway, and for which the limited capacity of a conventional 48 channel transceiver would be inadequate. In other instances, relatively large, secure, and protective structures for multiple racks of equipment are not necessarily available or cost effective, such as in PCS applications.
One way to resolve this difficulty would be to implement the basestation transceiver apparatus using a high speed analog-to-digital (A/D) converter and equipment which makes use of efficient digital filters. On the transmit side, the basestation would also include an inverse FFT processing combiner which outputs a combined signal representative of the contents of the communication channel signals processed thereby. In this manner, relatively compact, lightweight, inexpensive, and reliable digital integrated circuits may be used to cover the entire channel capacity offered by the service provider, rather than only the subset of the available channels. For an more detailed description of such a system, please refer to our co-pending United States patent application entitled "Transceiver Apparatus Employing Wideband FFT Channelizer with Output Sample Timing Adjustment and Inverse FFT Combiner for a Multichannel Communication Network" filed Apr. 10, 1994 and which is assigned to Overture Systems, Inc. the assignee of this application.
In such a configuration, on the receiver side, an antenna feeds a radio frequency tuner which selects an appropriately-sized bandwidth from among the radio frequency bandwidth available to the service provider. The analog tuner typically comprises one or more bandpass filters, amplifiers, local oscillators, and mixers to translate the selected bandwidth to a convenient center frequency at or near a baseband frequency.
The translated baseband signal is then amplified and then digitized by a high speed A/D converter. The A/D converter is characterized by a sampling rate and a digital word size, or resolution. The sampling rate is selected according to the bandwidth to be covered by the receiver, and a minimum sampling rate is at least twice the bandwidth to be covered, as dictated by the well-known Nyquist criterion. The word size is selected depending upon the desired sensitivity of the receiver. The greater the number of bits in each digital word output by the A/D, the greater the sensitivity, or dynamic range.
Current day state of the art circuit technologies typically limit the resolution of the A/D converter to approximately 12 bits at a sampling rate of 25 MegaHertz (MHz). This 12-bit converter thus provides a dynamic range of 72 to 80 decibels (dB) at most. Because analog bandpass filters, amplifier, and mixers are readily available which have a much greater dynamic range, wideband receivers of this type are thus considered to be dynamic range-limited by the A/D converter. This creates a number of problems.
First, the above dynamic range specification is for a signal of a single frequency of predictable maximum amplitude. However, in an application such as a wideband basestation, many signals of different amplitudes are present. Thus, the gain of the amplifier stage prior to the A/D converter must be carefully controlled, so that channel signals having the largest expected magnitude will be received without distortion. If this is not done, the resulting "clipping" of the received signal will create many undesired spurious tones. These spurious tones, in turn, cannot otherwise be separated from the desired lower magnitude signals in adjacent channels.
On the other hand, if the gain of the amplifier is reduced too much, in an effort to avoid the spurious sidelobe effect, this reduces the available dynamic range in the wideband receiver, and the smaller-amplitude signals may fall below the noise floor of the A/D converter, and thus may not be detected at all.
The conventional wisdom is thus that one must either accept a limit on the dynamic range of a wideband digital receiver which makes use of a given A/D converter, for a given basestation bandwidth. Otherwise, one must reduce the bandwidth covered by the digital receiver, so that fewer channels may be processed by a slower-speed A/D converter having a larger number of bits.
What is needed is a way to increase the dynamic range available in a wideband digital basestation, without reducing the bandwidth covered thereby. Indeed, it would be preferable if the receiver could somehow be designed such that this dependency of the bandwidth coverage on the dynamic range of the A/D converter could be eliminated.